Cooling water scale and corrosion inhibition

ABSTRACT

A methods for inhibiting silica scale formation and corrosion in aqueous systems where soluble silica (SiO 2 ) can be maintained at residuals below 200 mg/L, but more preferably maintained at greater than 200 mg/L as SiO 2 , without silica scale and with control of deposition of source water silica accumulations as high as 4000 mg/L (cycled accumulation) from evaporation and concentration of source water. The methods of the present invention also provide highly effective inhibition of corrosion for carbon steel, copper, copper alloy, and stainless steel alloys. The methods of the present invention comprise pretreatment removal of hardness ions from the makeup source water, maintenance of electrical conductivity, and elevating the pH level of the aqueous environment. Thereafter, specified water chemistry residual ranges are maintained in the aqueous system to achieve inhibition of scale and corrosion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication (Ser. No. not yet assigned) filed on Jan. 9, 2004, entitledCooling Water Scale and Corrosion Inhibition.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

Silica is one of the major scale and fouling problems in many processesusing water. Silica is difficult to deal with because it can assume manylow solubility chemical forms depending on the water chemistry and metalsurface temperature conditions. Below about pH 9.0, monomeric silica haslimited solubility (125-180 mg/L as SiO₂) and tends to polymerize asthese concentrations are exceeded to form insoluble (amorphous)oligomeric or colloidal silica. At higher pH, particularly above aboutpH 9.0, silica is soluble at increased concentrations of the monomericsilicate ion or in the multimeric forms of silica. Since conversion canbe slow, all of these forms may exist at any one time. The silicate ioncan react with polyvalent cations like magnesium and calcium commonlypresent in process waters to produce salts with very limited solubility.Thus it is common for a mixture of many forms to be present: monomeric,oligomeric and colloidal silica; magnesium silicate, calcium silicateand other silicate salts. In describing this complex system, it iscommon practice to refer to the mixture merely as silica or as silicaand silicate. Herein these terms are used interchangeably.

To address such problem, methods for controlling deposition and foulingof silica or silicate salts on surfaces in a aqueous process have beenderived and include: 1) inhibiting precipitation of the material fromthe process water; 2) dispersing precipitated material after it hasformed in the bulk water; 3) maintaining an aqueous chemical environmentthat supports formation of increased residuals of soluble silicaspecies; and 4) producing a non-adherent form of silica precipitants inthe bulk water. The exact mechanism by which specific scale inhibitionmethods of the present inventions function is not well understood.

In industrial application, most scale and corrosion control methods usedin aqueous systems typically rely on the addition of a scale andcorrosion inhibitor in combination with controlled wastage of systemwater to prevent scale and corrosion problems. In this regard, the majorscale formation potentials are contributed by the quantity of hardness(calcium and magnesium) and silica ions contributed by the source water,while the major corrosive potential results from the ionic orelectrolytic strength in the system water.

Treatment methods to minimize corrosion have further generally relied onthe addition of chemical additives that inhibit corrosion throughsuppression of corrosive reactions occurring at either the anode or thecathode present on the metal surface, or combinations of chemicaladditives that inhibit reactions at both the anode and cathode. The mostcommonly applied anodic inhibitors include chromate, molybdate,orthophosphate, nitrite and silicate whereas the most commonly appliedcathodic inhibitors include polyphosphate, zinc, organic phosphates andcalcium carbonate.

In view of toxicity and environmental concerns, the use of highlyeffective heavy metal corrosion inhibitors, such as chromate, have beenstrictly prohibited and most methods now rely on a balance of the scaleformation and corrosive tendencies of the system water and are referredto in the art as alkaline treatment approaches. This balance, as appliedin such treatment approaches, is defined by control of system waterchemistry with indices such as LSI or Ryznar, and is used in conjunctionwith combinations of scale and corrosion inhibitor additives to inhibitscale formation and optimize corrosion protection at maximumconcentration of dissolved solids in the source water. These methods,however, are still limited by the maximum concentration of silica andpotential for silicate scale formation. Moreover, corrosion rates arealso significantly higher than those available with use of heavy metalssuch as chromate. Along these lines, since the use of chromate and othertoxic heavy metals has been restricted, as discussed above, corrosionprotection has generally been limited to optimum ranges of 2 to 5 milsper year (mpy) for carbon steel when treating typical source waterqualities with current corrosion control methods. Source waters that arehigh in dissolved solids or are naturally soft are even more difficultto treat, and typically have even higher corrosion rates.

In an alternative approach, a significant number of methods of thepresent inventions for controlling scale rely on addition of acid totreated systems to control pH and reduce scaling potentials at higherconcentrations of source water chemistry. Such method allows forconservation of water through modification of the concentrated sourcewater, while maintaining balance of the scale formation and corrosivetendencies of the water. Despite such advantages, these methods have thedrawback of being prone to greater risk of scale and/or corrosionconsequences with excursions with the acid/pH control system. Moreover,there is an overall increase in corrosion potential due to the higherionic or electrolytic strength of the water that results from additionof acid ions that are concentrated along with ions in the source water.Lower pH corrosion control methods further rely on significantly higherchemical additive residuals to offset corrosive tendencies, but arelimited in effectiveness without the use of heavy metals. Silicaconcentration must still be controlled at maximum residuals by systemwater wastage to avoid potential silica scaling.

In a further approach, source water is pretreated to remove hardnessions in a small proportion of systems to control calcium and magnesiumscale potentials. These applications, however, have still relied oncontrol of silica residuals at previous maximum guideline levels throughwater wastage to prevent silica scale deposits. Corrosion protection isalso less effective with softened water due to elimination of thebalance of scale and corrosion tendency provided by the natural hardnessin the source water.

Accordingly, there is a substantial need in the art for methods that areefficiently operative to inhibit corrosion and scale formation that donot rely upon the use of heavy metals, extensive acidification and/orwater wastage that are known and practiced in the prior art. There isadditionally a need in the art for such processes that, in addition tobeing efficient, are extremely cost-effective and environmentally safe.Exemplary of those processes that would likely benefit from such methodswould include cooling water processes, cooling tower systems,evaporative coolers, cooling lakes or ponds, and closed or secondarycooling and heating loops. In each of these processes, heat istransferred to or from the water. In evaporative cooling waterprocesses, heat is added to the water and evaporation of some of thewater takes place. As the water is evaporated, the silica (or silicates)will concentrate and if the silica concentration exceeds its solubility,it can deposit to form either a vitreous coating or an adherent scalethat can normally be removed only by laborious mechanical or chemicalcleaning. Along these lines, at some point in the above processes, heatis extracted from the water, making any dissolved silicate less solubleand thus further likely to deposit on surfaces, thus requiring removal.Accordingly, a method for preventing fouling of surfaces with silica orsilicates, that further allows the use of higher levels ofsilica/silicates for corrosion control would be exceptionallyadvantageous. In this respect for cooling water, an inhibition methodhas long been sought after that would enable silica to be used as anon-toxic and environmentally friendly corrosion inhibitor.

To address these specific concerns, the current practice in theseparticular processes is to limit the silica or silicate concentration inthe water so that deposition from these compounds does not occur. Forexample in cooling water, the accepted practice is to limit the amountof silica or silicates to about 150 mg/L, expressed as SiO₂. Reportedly,the best technology currently available for control of silica orsilicates in cooling water is either various low molecular weightpolymers, various organic phosphate chemistries, and combinationsthereof. Even with use of these chemical additives, however, silica isstill limited to 180 mg/L in most system applications. Because in manyarid areas of the U.S. and other parts of the world make-up water maycontain from 50-90 mg/L silica, cooling water can only be concentrated 2to 3 times such levels before the risk of silica or silicate depositionbecomes too great. A method that would enable greater re-use or cyclingof this silica-limited cooling water would be a great benefit to theseareas.

SUMMARY OF THE INVENTION

The present invention specifically addresses and alleviates theabove-identified deficiencies in the art. In this regard, the inventionrelates to methods for controlling silica and silicate fouling problems,as well as corrosion of system metallurgy (i.e. metal substrates) inaqueous systems with high concentrations of dissolved solids. Moreparticularly, the invention is directed to the removal of hardness ionsfrom the source water and control of specified chemistry residuals inthe aqueous system to inhibit deposition of magnesium silicate and othersilicate and silica scales on system surfaces, and to inhibit corrosionof system metallurgy. To that end, we have unexpectedly discovered thatthe difficult silica and silicate scaling problems that occur in aqueoussystems when silica residuals exceed 125 mg/L, and more preferably areapproaching or greater than 200 mg/L as SiO₂, to as high as 4000 mg/L ofsilica accumulation (cycled accumulation from source water), can becontrolled by initially removing hardness ions (calcium and magnesium)from the makeup source water (i.e., water fed to the aqueous system)using pretreatment methods of the present inventions known in the art,such as through the use of ion exchange resins, selective ion removalwith reverse osmosis, reverse osmosis, electrochemical removal, chemicalprecipitation, or evaporation/distillation. Preferably, the pretreatmentmethods of the present invention will maintain the total hardness in themakeup water at less than 20% of the makeup silica residual (mg/L SiO₂),as determined from an initial assessment of the source water. In someembodiments, the total hardness ions will be maintained at less than 5%of the makeup silica residual. When source makeup water is naturallysoft, with less than 10 mg/L hardness as CaCO₃, pretreatment removal ofhardness ions may be bypassed in some systems. Thereafter, theconductivity (non-neutralized) in the aqueous system is controlled suchthat the same is maintained at some measurable level (i.e., at least 1μmhos and the pH of the source water elevated to a pH of approximately9.0, and preferably 9.6, or higher. With respect to the latter, the pHmay be adjusted by the addition of an alkaline agent, such as sodiumhydroxide, or by simply removing a portion of the aqueous system waterthrough such well known techniques or processes as evaporation and/ordistillation.

In a related application, we have unexpectedly discovered that theexcessive corrosion of carbon steel, copper, copper alloys, andstainless steel alloys in aqueous systems due to high ionic strength(electrolytic potential) contributed by dissolved solids source water orhighly cycled systems can likewise be controlled by the methods of thepresent inventions of the present invention. In such context, themethods of the present invention comprises removing hardness ions(calcium and magnesium) from the makeup source water using knownpretreatment methods of the present inventions, such as ion exchangeresins, selective ion removal with reverse osmosis, reverse osmosis,electrochemical removal, chemical precipitation, orevaporation/distillation. The pretreatment methods of the presentinvention will preferably maintain the total hardness ratio in themakeup water at less than 20%, and preferably at least less than 5%, ofthe makeup silica residual (mg/L SiO₂), as determined from an initialanalysis of the source water. When source makeup water is naturallysoft, with less than 10 mg/L hardness as CaCO₃, pretreatment removal ofhardness ions may be bypassed in some systems. Thereafter, theconductivity (non-neutralized) in the aqueous system is controlled suchthat the same is maintained at some measurable level (i.e., at least 1μmhos). Alkalinity is then controlled as quantified by pH at 9.0 orhigher, with a pH of 9.6 being more highly desired in some applicationsalong with control of soluble silica at residual concentrationsapproaching or exceeding 200 mg/L, but not less than 10 mg/L, withcontrol at more highly desired residuals in some applicationsapproaching or exceeding 300 mg/L as SiO₂. With respect to the latter,the SiO₂ may be adjusted by the addition of a silica/silicate agent,such as sodium silicate, or by simply removing a portion of the aqueoussystem water through such well known techniques or processes asevaporation and/or distillation.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below is intended as a description ofthe presently preferred embodiment of the invention, and is not intendedto represent the only form in which the present invention may beconstructed or utilized. The description sets forth the functions andsequences of steps for constructing and operating the invention. It isto be understood, however, that the same or equivalent functions andsequences may be accomplished by different embodiments and that they arealso intended to be encompassed within the scope of the invention.

According to the present invention, there is disclosed methods forinhibiting silica and silicate scale in aqueous systems and providingexceptional metal corrosion protection that comprise the removal ofhardness from the makeup source water prior to being fed into theaqueous system and thereafter controlling the aqueous system withinspecified water chemistry control ranges. Specifically, hardness ions(calcium and magnesium) are removed from the makeup source water usingpretreatment methods known in the art, which include methods such as ionexchange resins, selective ion removal with reverse osmosis, reverseosmosis, electrochemical removal, chemical precipitation, orevaporation/distillation. Multivalent metal ions such as those fromiron, copper, zinc, barium, and aluminum are usually at lowconcentrations in treated municipal and well source waters used for makeup to cooling systems. These low level concentrations will not typicallyrequire removal if the total concentration of these metals in additionto hardness ions (calcium and magnesium) following pretreatment arebelow the maximum ratio specified based on source water silica residual.However, some water sources such as well, reclaimed or untreated surfacewaters may have higher residuals of these metals as well as otherobjectionable materials. Such waters may require pretreatment withalternative methods for reduction of these multivalent metal ions inaddition to the pretreatment methods specified by the method for removalof calcium and magnesium multivalent metal ions.

The pretreatment methods will preferably maintain the total hardnessratio in the makeup water at less than 20% of the makeup silica residual(mg/L SiO₂). In a more highly preferred embodiment, the pretreatmentmethods will maintain the total hardness ions present in the makeupwater at less than 5% of the makeup silica residual. As will beappreciated by those skilled in the art, the silica residual can bereadily determined by utilizing known techniques, and will preferably bedetermined prior to the application of the methods of the presentinvention. Along these lines, when source makeup water is naturallysoft, with less than 10 mg/L hardness as CaCO₃, pretreatment removal ofhardness ions may be bypassed in some systems.

Conductivity (non-neutralized) is established in the aqueous system suchthat at least some measurable conductivity is present, which is definedas at least 1 μmhos and preferably at least 500 μmhos. Control ofconductivity may be conducted through control or elimination of blowdownwastage from the system. In a more highly preferred embodiment,conductivity will be maintained between 10,000 and 150,000 μmhos.Conductivity levels attained in method treated systems will depend uponsystem capability to concentrate source water, level of dissolved solids(conductivity) in the pre-treated or natural source water, and potentialaddition of adjunct alkalinity or chemical to attain required controlresiduals. The higher level of ionic strength in the more highlypreferred embodiment control range of 10,000 to 150,000 μmhos willincrease the solubility of multivalent metal salts that are less solubleat lower ionic strengths of other methods of the present inventions.This residual control parameter also provides indirect control of silicaand alkalinity (pH) residuals contributed by concentration of availablesilica and alkalinity in the pre-treated or natural source water or byaddition of adjunct forms of these chemicals.

Aqueous system pH is maintained at 9.0 or greater as contributed by thecycled accumulation of alkalinity from the source water or throughsupplemental addition of an alkalinity adjunct, such as sodiumhydroxide, to the system when required. The minimum pH will provideincreased solubility of silica and control of silicate scale and supportcorrosion protection for metals. Along these lines, in certain preferredembodiments of the present invention, the pH may be raised andmaintained to a level of 9.6 of higher.

To support corrosion inhibition, soluble silica residuals willpreferably be maintained in the aqueous system at levels approaching orexceeding 200 mg/L, but not less than 10 mg/L, as contributed by thecycled accumulation of silica from the source water or throughsupplemental addition of adjunct forms of silica to the system whenrequired. In certain applications, such levels may be maintained atlevels of greater than 300 mg/L. A 200 mg/L minimum residual of solublesilica will support corrosion inhibition for metals, and moreparticularly, inhibit corrosion of carbon steel to less than 0.3 mpy andless than 0.1 mpy for copper, copper alloys and stainless steel alloyspresent in the aqueous system. The method will control carbon steelcorrosion at less than 5 mpy (less than 0.3 mpy for copper) in treatedsystems controlled at silica residuals less than 200 mg/l (as SiO₂),with reduction of source water multivalent metal ions (hardness) tospecified residuals and pH control at 9.0 or greater.

With respect to the mechanisms by which the methods of the presentinventions effectively achieve their results, excess source water silica(beyond the soluble residuals attained with specified pH control) isprobably adsorbed as non-adherent precipitates that form followingreaction with small amounts of metals (Ca, Mg, Fe, Al, Zn) or solidsintroduced by source water or scrubbed from the air by the tower system.This is the probable result of the expanded solubility of the monomericand multimeric species of silica with the methods of the presentinvention that impede polymerization of excess silica until it reactswith these incrementally introduced adsorption materials to form smallquantities of non-adherent precipitants. The adsorption andprecipitation of high ratios of silica on small amounts of solids suchas magnesium hydroxide has been demonstrated by the Freundlichisotherms, and is common experience in water treatment chemicalprecipitation processes. The small quantity of precipitate is removedfrom the circulating water through settling in the tower basin or driftlosses.

Control of the lower solubility hardness scale formations and resultantnucleation sites on cooling system surfaces are controlled with themethods disclosed herein, through pretreatment removal of the majorityof the scale forming (hardness) metal ions and control of system waterat the specified higher ionic strength control ranges. The higher levelof ionic strength in the preferred control range increases thesolubility of scale forming metal salts. Such approach is well suited toaddress a further complication in controlling silica and silicatefouling brought about from the phenomena that colloidal silica tends tobe more soluble as temperature is raised, while the polyvalent metalsalts of the silicate ion tend to be less soluble with increasingtemperature. As a result, control or minimization of polyvalent metalsin the aqueous solution will prohibit formation of the insoluble saltson heat transfer surfaces, and promote increased solubility of otherforms of silica at the elevated temperatures of heat transfer surfaces.The present methods thereby eliminate potential reaction of insolublesilica forms with hardness scale or metal salt deposits on systemsurfaces and their nucleation sites that initiate silica or silicatescale formations. The method will control silica scale formation intreated systems with silica residuals exceeding those permitted by priorart (maximum solubility 125 to 180 mg/l monomeric silica), withreduction of source water multivalent metal ions (hardness) to specifiedresiduals and pH control at 9.0 or higher.

The higher residuals of soluble silica and higher pH levels maintainedvia the present methods of the present inventions provide highlyeffective polarization (corrosion barrier formation) and exceptionalcorrosion protection for carbon steel, copper, copper alloy andstainless steel metals (less than 0.3 mpy for mild steel, and less than0.1 mpy copper, copper alloy, and stainless steel). Moderately highercorrosion rates may be acceptable to end users when low silica sourcewaters do not permit attainment of residuals approaching or exceeding200 mg/l SiO₂ in the method treated water at the system's maximumattainable source water concentrations. Such moderately elevatedcorrosion levels are superior or equivalent to current art. Comparablecorrosion rates for carbon steel in aqueous systems with existingmethods of the present inventions are optimally in the range of 2 to 5mpy. When pH is increased to levels higher than 9.0, and residuals ofsilica are increased, approaching 200 mg/l SiO₂, corrosion levels willbe reduced to those levels disclosed in Applicants' co-pending patentapplication (Ser. No. not yet assigned), the teachings of which areincorporated herein by reference. Maximum attainable source waterconcentrations may be limited by low evaporative load and/oruncontrollable system water losses (such as tower drift). If the enduser does require lower corrosion rates, such results are attainable bysupplemental addition of adjunct silica to the cooling water to provideresiduals approaching or greater than 200 mg/L SiO₂.

Though not fully understood, several corrosion inhibition mechanisms arebelieved to be contributing to the metals corrosion protection providedby the methods of the present invention, and the synergy of both anodicand cathodic inhibition functions may contribute to the corrosioninhibition process. Control at lower silica residuals probably reducesthe effectiveness of corrosion inhibition due to reduction of availablemonomeric silica and converted multimeric forms of silica that provideanodic corrosion inhibition to metals with the method. Higherconcentrations of silica and higher pH levels will provide increasedmultimeric silica residual concentrations for optimum anodic protectionafforded by the method. Operating at lower soluble silica concentrationswill also reduce the corrosion inhibition effectiveness of methodtreated systems if pretreatment upsets lead to elevated hardness levelsin the source water exceeding those specified in the method, since highsource water residuals of hardness salts can then more easily absorb anddeplete the reduced multimeric silica residuals formed by the method atlow silica residual conditions.

In this regard, an anodic corrosion inhibitor mechanism results fromincreased residuals of soluble silica provided by the present methods,particularly in the multimeric form. Silicates inhibit aqueous corrosionby hydrolyzing to form negatively charged colloidal particles. Theseparticles migrate to anodic sites and precipitate on the metal surfaceswhere they react with metallic ion corrosion products. The result is theformation of a self-repairing gel whose growth is self-limited throughinhibition of further corrosion at the metal surface. Unlike themonomeric silica form normally found in source water that fails toprovide effective corrosion inhibition, the methods of the presentinvention provide such beneficial effect by relying upon the presenceand on control of total soluble silica residuals, with conversion ofnatural monomeric silica to the multimeric forms of silica at muchhigher levels, through application of the combined control ranges as setforth above. In this respect, the removal of most source water calciumand magnesium ions is operative to prevent reaction and adsorption ofthe multimeric silica forms on the metal oxide or metal saltprecipitates from source water, which is believed to be an importantcontribution to the effectiveness of this corrosion inhibition mechanismafforded by the present invention. The resultant effective formation andcontrol of the multimeric silica residuals with such methods of thepresent invention has not heretofore been available.

In addition to an anodic corrosion inhibition mechanism, a cathodicinhibition mechanism is also believed to be present. Such inhibition iscaused by an increased hydroxyl ion concentration provided with thehigher pH control range utilized in the practice of the presentinvention. In this regard, iron and steel are generally consideredpassive to corrosion in the pH range of 10 to 12. The elevated residualof hydroxyl ions supports equilibrium with hydroxyl ion produced duringoxygen reduction at the cathode, and increases hydroxyl ion availabilityto react with iron to form ferrous hydroxide. As a consequence, ferroushydroxide precipitates form at the metal surface due to very lowsolubility. The ferrous hydroxide will further oxidize to ferric oxide,but these iron reaction products remain insoluble at the higher pHlevels attained by implementing the methods described herein to polarizeor form a barrier that limits further corrosion. At the 9 to 10 pH range(as utilized in the practice of the present invention), effectivehydroxyl ion passivation of metal surfaces may be aided by thepretreatment reduction of hardness ions (calcium and magnesium) in thesource water that may compete with this reaction and interfere withmetal surface barrier formation.

Galvanized steel and aluminum may be protected in general by thesilicate corrosion inhibitor mechanism discussed herein, but protectivefilms may be destabilized at water-air-metal interfaces. Steel, copper,copper alloy, stainless steel, fiberglass, and plastic are thus idealaqueous system materials for application of the methods of the presentinventions of the present invention.

The extensive improvement in corrosion protection provided by themethods of the present invention is not normally attainable with priorart methods when they utilize significantly higher residuals ofaggressive ions (e.g., chloride and sulfate) and the accompanyinggreater ionic or electrolytic strength present in the aqueous systemwater. This may result from either use of acid for scale control and/orconcentration of source water ions in the aqueous system. As is known,corrosion rates generally increase proportionately with increasing ionicstrength. Accordingly, through the ability to protect system metalsexposed to this increased electrolytic corrosion potential, opportunityfor water conservation and environmental benefits that result withelimination of system discharge used with previous methods to reducecorrosion or scaling problems in aqueous systems can be readily realizedthrough the practice of the methods disclosed herein. Indeed,significant water conservation can still be obtained with the method,even operating with silica residuals less than 200 mg/L, throughelimination of blowdown wastage and subsequent concentration of sourcewater dissolved solids (conductivity) to higher levels, without silicatescale formation or excessive corrosion.

Still further, the methods of the present inventions of the presentinvention can advantageously provide gradual removal of hardness scaledeposits from metal surfaces. This benefit is accomplished through bothpretreatment removal of the majority of the scale forming (hardness)metal ions and control of system water at the specified higher ionicstrength control ranges. Solubility of hardness salts is increased bythe higher ionic strength (conductivity) provided by the present methodsof the present invention, which has been determined with high solidswater such as seawater, and may contribute to the increased solubilityof deposits present within the aqueous environment so treated. Studiesconducted with hardness scale coated metal coupons in treated systemsdemonstrated a significant deposit removal rate for CaCO₃ scale films inten days. Control of source water hardness at lower specified residualswill probably be required to achieve optimum rate of hardness scaleremoval.

Furthermore, the present methods advantageously prohibits microorganismpropagation due to the higher pH and dissolved solids levels that areattained. Biological fouling potentials are thus significantly reduced.In this regard, the methods of the present inventions disclosed hereincreate a chemical environment that inhibits many microbiological speciesthat propagate at the pH and dissolved solids chemistry ranges used withprevious treatment approaches. The reduction in aqueous system dischargeas further provided as a by product of the present invention alsopermits use of residual biocides at more effective and economicaldosages that impede development of problem concentrations of anymicrobiological species that are resilient in the aqueous environmentgenerated through the practice of the methods of the present inventionsdisclosed herein.

A still further advantage of the methods of the present inventioninclude the ability of the same to provide a lower freeze temperature inthe aqueous system, comparable to ocean water, and avert potentialmechanical damage from freezing and/or operational restrictions forsystems located in freeze temperature climates.

Additional modifications and improvements of the present invention mayalso be apparent to those of ordinary skill in the art. Thus, theparticular combination of parts and steps described and illustratedherein is intended to represent only certain embodiments of the presentinvention, and is not intended to serve as limitations of alternativedevices and methods of the present inventions within the spirit andscope of the invention. For example, since the methods of the presentinvention provides both effective silicate scale control and corrosioninhibition when using high silica or high dissolved solids sourcewaters, extensive variation in source water quality can be tolerated.These source waters might otherwise be unacceptable and uneconomical foruse in such aqueous systems. In addition, such modifications mayinclude, for example, using other conventional water treatment chemicalsalong with the methods of the present invention, and could include otherscale inhibitors, such as for example phosphonates, to control scalesother than silica, corrosion inhibitors, biocides, dispersants,defoamers and the like. As will be appreciated, however, control atlower conductivity levels may reduce the effectiveness of the method inremoving existing hardness deposits, lowering of system water freezetemperature, and prohibition of microorganism propagation. Accordingly,the present invention should be construed as broadly as possible.

As an illustration, below there are provided non-restrictive examples ofan aqueous water system that has been treated with methods conforming tothe present invention.

EXAMPLES OF SILICATE SCALE INHIBITOR METHOD

The following analytical tests were performed on a cooling tower systemtreated with the methods of the present invention to demonstrate theefficacy of the present invention for controlling the solubility ofsilica and silicate species, and preventing scale deposition of thesespecies. Two samples of each of the following: 1) varying source water;2) the resultant treated system water; and 3) tower sump insolubleaccumulations, for a total of six samples were analyzed from differentoperating time frames.

Although the exact mechanism of action of the process is not completelyunderstood, the methods of the present invention minimize the turbidityof the treated water, which is considered a demonstration of aneffective silica and silicate scale inhibitor. Methods that producetreated water of less than eight nephelometric turbidity units (NTU) areconsidered improvements over the current available technology. Turbiditymeasurements (Table 1) performed on samples taken from the coolingsystems, before and after filtration through a 0.45-micron filter,illustrate effective silicate inhibition in the treated water. Theturbidity levels are well below typical cooling tower systems, inparticular at such high concentrations (80 COC), and indicate themethods of the present invention provide controlled non-adherentprecipitation of excess silica and other insoluble materials enteringthe system. Clean heat exchanger surfaces have confirmed that the methodsilica precipitation is non-adherent. The precipitated silica forms arecontained in the cooling tower sump. However, the volume of precipitantand scrubbed accumulations in the tower sump were not appreciablygreater than previous treatment methods due to reduction of insolublemultivalent metal salt precipitates by pretreatment removal. TABLE 1Tower Water Turbidity Analyses Sample No. 1: (Turbidity, NTU) Neat, 4NTU; Filtered, 2 NTU Sample No. 2: (Turbidity, NTU) Neat, 3 NTU

The cooling tower and makeup water analytical tests performed in Table 2and Table 3 illustrate the effectiveness of the methods of the presentinvention in maintaining higher levels of soluble silica in the coolingtower system when parameters are controlled within the specified pH andlow makeup hardness ranges. Soluble silica residuals are present at 306and 382 mg/L in these tower samples at the respective 9.6 and 10.0 pHlevels. The lower cycles of concentration (COC) for silica in thesetower samples, as compared to the higher cycled residuals for solublechemistries (chloride, alkalinity, conductivity), indicate that excesssilica is precipitating as non-adherent material, and accumulating inthe tower basin. This is confirmed by the increased ratio of silicaforms found in tower basin deposit analyses. System metal and heatexchange surfaces were free of silica or other scale deposits. TABLE 2Cooling Tower Sample No. 1/Makeup/Residual Ratios (COC) MakeupSAMPLE/TESTS Tower (soft) COC Conductivity, 33,950 412 82.4 μmhos(Un-neutralized) pH 10.01 8.23 NA Turbidity, NTUs Neat 3 0.08 NAFiltered (0.45μ) — — — Copper, mg/L Cu ND ND NA Zinc, mg/L ND ND NASilica, mg/L SiO₂ 382 9.5 40.2 Calcium, mg/L 16.0 0.20 NA CaCO₃Magnesium, mg/L 3.33 0.05 NA CaCO₃ Iron, mg/L Fe ND ND NA Aluminum, mg/LAl ND ND NA Phosphate, mg/L ND ND NA PO₄ Chloride, mg/L 6040 80 75.5Tot. Alkalinity, 13200 156 84.6 mg/LND = Not Detectable;NA = Not Applicable;COC = Cycles of Concentration

TABLE 3 Cooling Tower Sample No. 2/Makeup/Residual Ratios (COC) MakeupSAMPLE/TESTS Tower (soft) COC Conductivity, 66,700 829 80 μmhos(Un-neutralized) pH 9.61 7.5 NA Turbidity, NTUs Neat 4 0.08 NA Filtered(0.45μ) 2 — — Zinc, mg/L ND ND NA Silica, mg/L SiO₂ 306.4 11 28 Calcium,mg/L 21.5 0.20 NA CaCO₃ Magnesium, mg/L 0.65 0.05 NA CaCO₃ Iron, mg/L FeND ND NA Aluminum, mg/L Al ND ND NA Phosphate, mg/L ND ND NA PO₄ND = Not Detectable;NA = Not Applicable;COC = Cycles of Concentration

Microscopic and chemical analysis of deposit samples from accumulatedresidue in the tower basin of a system treated by present methodologyare shown in Exhibit 1 and Exhibit 2. Both analyses illustrate thesignificant ratio of silica materials in the deposit. The majorproportion of this silica is the probable result of silica adsorption orreaction with insoluble precipitates of multivalent metals as theyconcentrated in the tower water. Visual inspections of heat transferequipment in the system treated by this method have confirmed that ithas remained free of silica and other scale deposits. System heattransfer efficiencies were also maintained at minimum fouling factorlevels.

Exhibit 1

MICROSCOPICAL ANALYSIS - POLARIZED LIGHT MICROSCOPY DEPOSIT DESIGNATION:Cooling Tower Basin Deposit % ESTIMATED CONSTITUENTS >30 Amorphoussilica, including assorted diatoms, probably including amorphousmagnesium silicate; calcium carbonate (calcite) 1-2 Assorted claymaterial including feldspar; hydrated iron oxide; carbonaceous material<1 Silicon dioxide (quartz); assorted plant fibers; unidentifiedmaterial including possibly aluminum oxide (corundum)

Exhibit 2

CHEMICAL ANALYSIS - DRIED SAMPLE DEPOSIT DESIGNATION: Cooling TowerBasin Deposit % ESTIMATED CONSTITUENTS 12.1 CaO 8.5 MgO 5.2 Fe₃O₄ 3.7Fe₂O₃ <0.5 Al₂O₃ 13.2 Carbonate, CO₂ 51.1 SiO₂ 5.7 Loss on IgnitionMost probable combinations: Silica ˜54%, Calcium Carbonate ˜32%, Oxidesof Iron ˜9%, Mg and Al Oxides ˜5%.

EXAMPLES OF CORROSION INHIBITION METHODS OF THE PRESENT INVENTION

The data in Table 4 illustrate the effectiveness of the methods of thepresent invention in inhibiting corrosion for carbon steel and coppermetals evaluated by weight loss coupons in the system. No pitting wasobserved on coupon surfaces. Equipment inspections and exchanger tubesurface testing have confirmed excellent corrosion protection.Comparable corrosion rates for carbon steel in this water quality withexisting methods of the present inventions are optimally in the range of2 to 5 mpy. TABLE 4 CORROSION TEST DATA Specimen Type Carbon SteelCopper Test location Tower Loop Tower loop Exposure period 62 Days 62Days Corrosion Rate (mpy) 0.3 <0.1

EXAMPLES OF SCALE DEPOSIT REMOVAL

The data in Table 5 illustrate harness (CaCO₃) scale removal from metalsurfaces in a tower system treated with the methods of the presentinvention through coupon weight loss reduction. Standard metal couponsthat were scaled with CaCO₃ film were weighed before and after ten daysof exposure and the visible removal of most of the scale thickness. Thedemonstrated CaCO₃ weight loss rate will provide gradual removal ofhardness scale deposits that have occurred in a system prior to methodtreatment. TABLE 5 SCALE DEPOSIT REMOVAL TEST DATA Specimen Type CarbonSteel Copper Test location Tower Loop Tower loop Exposure period 10 Days10 Days Scale Removal (mpy) 8.3 8.1

1. A method for controlling silica or silicate scale formation in anaqueous heat transfer water system with silica contributed by sourcewater, the methods of the present invention comprising the steps: a)removing hardness ions from said source water; b) controlling theconductivity of said aqueous system water such that said aqueous systemwater possesses a measurable conductivity of at least 1 μmhos; c)elevating and maintaining the pH of said aqueous system water such thatsaid aqueous system water possesses a pH of approximately 9.0 orgreater; and d) providing a metallic heat transfer surface andcyclically contacting said aqueous system water thereabout.
 2. Themethod of claim 1 wherein in step a), said hardness ions comprise ionsof calcium and magnesium.
 3. The method of claim 1 wherein said aqueoussystem water contains soluble SiO₂ residual that is at least 125 mg/L.4. The method of claim 3 wherein said aqueous system water containssoluble SiO₂ in excess of 200 mg/L.
 5. The methods of the presentinvention of claim 3 wherein in step a), said hardness ions are removedin amounts equal to or less than approximately 20% of the SiO₂ presentwithin said source water.
 6. The methods of the present invention ofclaim 3 wherein in step a), said hardness ions are removed in amountsequal to or less than approximately 5% of the SiO₂ present within saidsource water.
 7. The method of claim 1 wherein in step c), said pH ismaintained at 9.6 or higher.
 8. The method of claim 1 wherein in stepa), said hardness ions are removed via a method selected from the groupconsisting of ion exchange, selective ion removal with reverse osmosis,reverse osmosis, electro chemical removal, chemical precipitation,evaporation and distillation.
 9. The method of claim 1 wherein in stepc), said pH is increased by adding an alkali agent.
 10. The method ofclaim 9 wherein said alkali agent comprises sodium hydroxide.
 11. Themethod of claim 1 wherein in step c), said pH is elevated by evaporatinga portion of said aqueous system water.
 12. The method of claim 1wherein in step c), said pH is elevated by distilling a portion of saidaqueous system water.
 13. The method of claim 1 wherein in step c), saidaqueous heat transfer water system comprises water utilized for coolingprocesses, water utilized for cooling tower systems, water utilized forevaporative cooling, water utilized for cooling lakes or ponds, waterutilized for enclosed or secondary cooling and heating loops. 14-26.(canceled)
 27. The method of claim 1 wherein prior to step a), saidmethods of the present invention comprises the step: a) analyzing saidsource water to determine the concentration of SiO₂ present therein. 28.(canceled)
 29. The method of claim 1 wherein in step b), saidconductivity of said aqueous system water is controlled such that saidaqueous system water possesses a conductivity of at least 500 μmhos. 30.(canceled)
 31. The method of claim 1, wherein said source water containssilica in an amount of 4000 mg/L or less.
 32. (canceled)